This invention relates to energy transfer machines. More specifically, this invention relates to energy transfer machines of the turbomachinery type wherein a working fluid and a turbomachine rotor are arranged for multiple stage energy transfer, and including apparatus and methods for controlling the level of energy exchange.
Energy transfer machines of the turbomachinery type are well known in the prior art, and typically comprise a multi-bladed rotor disposed in a fluid passage for coaction with a suitable working fluid. In one configuration, the rotor is rotatably driven to impart flow energy to the fluid. Alternately, in the case of a moving fluid, flow energy of the fluid may be used to impart rotational energy to the rotor.
In the prior art, fluid energy transfer machines have been adapted for multiple stages of energy transfer between a fluid and a single rotor in order to obtain a relatively high specific work output. More specifically, transfer machines have been developed wherein the fluid is circulated for contact and energy transfer with the blade cascade of a rotor more than one time. These machines typically involve recirculating of the fluid to the rotor via a return duct, or include stator vanes for readmission of the fluid to the rotor for second and subsequent energy transfers. However, these reentry type machines have a number of disadvantages including the presence of interstage leakage and the restriction of fluid flow to a single flow path for all conditions of machine loading.
Other prior art devices have been developed which also attempt multiple stages of energy transfer with a single rotor, but without reentry type return ducts or the like. However, these devices typically do not provide close fluid flow control, but instead allow the fluid to move in and out of rotor contact generally in a disorderly and uncontrolled fashion, primarily for lack of stator vane guidance. This disorderliness of flow results in inefficient energy transfer and undesirable machine back pressures.
U.S. Pat. No. 3,292,899, issued Dec. 20, 1966 to Hans Egli, one of the co-inventors of the present invention, discloses an energy transfer machine constituting a major improvement over the prior art in general. The energy transfer machine of this patent includes a casing with a fluid inlet and outlet, and a blade cascade on a rotor arranged for movement past the inlet and outlet. The casing is devoid of stator vanes, and is configured so that a fluid passing from the inlet to the outlet is constrained to flow in a generally spiralling path in association a number of times with the blade cascade. The geometry of the fluid path is primarily dictated by the shape of the casing, the geometry of introduction of the fluid into the housing, and the pressure gradient on the fluid which is related to the machine back pressure. For example, if the back pressure is increased for a given rotor speed, the number of passes of the fluid through the blade cascade increases, with the fluid flow pattern remaining smooth and orderly, to increase the level of energy transfer between the rotor and fluid. Importantly, for a practical level of energy exchange to occur, the fluid should pass at least twice through the blade cascade.
U.S. Pat. No. 3,782,850, issued Jan. 1, 1974 to Hans Egli, Fredrick E. Burdette, and James H. Nancarrow, two of whom are co-inventors of the present invention, discloses an energy transfer machine designed to enhance the performance and the structural simplicity of the machine disclosed in U.S. Pat. No. 3,292,899. The machine includes a casing forming a toroidal volume generally concentrically enclosing a stator ring. The ring and casing define a circumferential fluid passage of generally annular cross-section extending between a fluid inlet and a fluid outlet. A rotor has a blade cascade in close running clearance with the stator ring to coact with fluid in the passage to cause the fluid to flow in a generally spiralling path about the stator ring, and thereby make a number of passes through the blade cascade to effect multiple stages of energy transfer.
In some energy transfer machine applications, it is desirable to control or prevent the flow of fluid through the machine. For example, when the machine is used as an air pump to provide supplemental emission control air to a combustion engine, it is desirable to disconnect or stop air flow when the supplemental air is not required. In the prior art, the most common method of preventing fluid flow comprises a throttling of the flow at the machine inlet or outlet. However, with energy transfer machines of the type disclosed, a throttling of fluid flow does not unload the pump rotor, but instead increases the back pressure on the machine. Such increases in back pressure increases the number of stages of energy transfer, and thereby increases the driving load on the rotor. Accordingly, prior art throttling schemes are not satisfactory for use with such energy transfer machines.
The present invention overcomes the problems and disadvantages of the prior art by providing an energy-efficient transfer machine including apparatus and methods for unloading the machine to substantially eliminate the pressure gradient between the machine fluid inlet and fluid outlet.